ci(drift): harden drift checks — promote to required status check + cross-repo guards (umbrella)

[saadqbal]

Follow-up to #218 (merged 2026-06-08).

The Source-of-truth drift job (.github/workflows/drift-checks.yaml → scripts/tests/check-drift.sh) now runs on PRs that touch scripts/** or client/**, but it only reports — it doesn't block. Once it's proven stable over a few PR cycles, promote it to a required status check so drift actually gates merges.

Acceptance

• Source-of-truth drift added to the required checks for develop (and main) in the repo ruleset
• Verified it blocks a PR that introduces deliberate drift (e.g. a renamed workload or a changed backend host)

Refs #218, tracebloc/backend#746

[LukasWodka]

Consolidating the two follow-ups to #218 into one umbrella:

1. Promote Source-of-truth drift to a required status check — this issue.
2. Cross-repo guards — docs selectors vs chart labels; preflight thresholds vs docs sizing — folded from ci(drift): cross-repo guards — docs selectors ↔ chart labels; preflight thresholds ↔ docs sizing #220.

Closing #220 in favor of this.